The expression and function of CD59, a 19-25 kDa membrane glycoprotein
that inhibits formation of the membrane attack complex of complement,
was analyzed on normal and malignant human colonic epithelial cells. A
nalysis by immuno-fluorescence demonstrated a weak apical expression o
f CD59 on normal intestinal epithelium, with an increased expression o
n adenocarcinoma cells. The expression of CD59 was greatest on tumor c
ells with poor differentiation. The functional activity of CD59 on hum
an adenocarcinoma cells was investigated using the colonic adenocarcin
oma cell line HT29. CD59 on HT29 cells was glycosyl-phosphatidylinosit
ol-linked, and had a molecular mass of 19-25 kDa. HT29 cells expressed
approximately four times more CD59 than leukocytes, and showed a high
resistance to antibody-dependent complement-mediated lysis. Blocking
of CD59 with divalent antigen-binding F(ab')(2) fragments of the anti-
CD59 monoclonal antibody 1F5 resulted in a dose-dependent increase in
complement-mediated lysis, suggesting that CD59 may be of importance i
n protecting colonic adenocarcinoma cells against complement-mediated
cytolysis.